1. Field of the Invention
This invention relates generally to semiconductor active and passive devices, such as photonic, electronic or optoelectronic devices or array waveguide gratings (AWGs) and other passive waveguides, and more particularly to waveguide structures in InP based components such as, for example, InP based array waveguide gratings (AWGs) and avalanche photodiodes (APDs).
2. Description of the Related Art
In the past, array waveguide gratings (AWGs), avalanche photodiodes (APDs) or other optical active and passive components that contain or include a waveguide core based in an InP regime or material system have utilized InGaAsP as a waveguide layer or waveguide core. The filter characteristics of these devices are extremely sensitive to any variations in the composition of Group III–V materials utilized to form the waveguide layer or core in such devices as the same is epitaxially formed with planarity across an InP wafer. An InGaAsP waveguide core is problematic due to the fact that significant composition variations occur across the wafer as the material is grown epitaxially, which is typically via a MOCVD reactor. As a result, longitudinally or transversely, these waveguide structures in the resulting optical or opto-electronic devices have these same variations along their paths, meaning that their refractive index may vary along their length and width. These variations are typically caused by variations in the As/P composition ratio and occur due to the fact that the source gases typically utilized to grow this material, e.g., AsH3 and PH3, have significantly different cracking efficiencies as a function of temperature. Consequentially, the photoluminescent (PL) emission wavelength, which directly relates to compositional variations, varies by about 10 nm to about 20 nm across the wafer for InGaAsP as-grown materials or layers. These composition variations wreak havoc on the performance and yield of InP-based AWGs and other optical components that utilize InGaAsP as the core waveguide layer. Furthermore, InGaAsP materials can also be difficult to target (reproducibility) from one epitaxial growth to the next, further affecting performance and yield.
Other group III–V waveguide materials and regimes have been utilized and are known in the art. The particular interest here is the InP regime where InAlGaAs provides a high refractive index for waveguiding properties as InGaAsP. The use of InAlGaAs material as a waveguide core in InP semiconductor devices is known. For example, see U.S. Pat. No. 5,910,012 and FIGS. 7A–7C directed to a photodetector structure and a spot size converter (SSC) or tapered section where InAlGaAs is suggested for the quantum well core layer at 7a, 7b and 7c. Also, in U.S. Pat. No. 5,689,358 and the description of FIGS. 47–49, it is indicated that a spot size converter (SSC) can be fabricated with a waveguide core of InAlGaAs. However, there is no teaching or appreciation in the art of utilizing InAlGaAs waveguide structures in photonic integrated circuits (PICs) where high wavelength uniformity is required, necessary and achieved across the wafer when deploying InAlGaAs in lieu of InGaAsP as the waveguide or core for the optical coupling path among plural active and passive optical components integrated in the PIC.
The uniformity across an as-grown wafer, such as an InP wafer, becomes much more important when multiple optical components and electro-optical components are together fabricated as a photonic integrated circuit (PIC) formed in multiple chip or die in the wafer because enhances the probability of achieving higher batch-to-batch reproducibility of such as-grown photonic integrated circuits.